Date: Sun, 11 May 2003 12:22:46 -0400 From: Don Bowman <don@sandvine.com> To: 'Luigi Rizzo' <rizzo@icir.org>, Don Bowman <don@sandvine.com> Cc: freebsd-net@freebsd.org Subject: RE: polling(4) and idle time/cpu usage percentages Message-ID: <FE045D4D9F7AED4CBFF1B3B813C8533701B36612@mail.sandvine.com>
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From: Luigi Rizzo [mailto:rizzo@icir.org]
> [Polling in SMP case]
>
> Interesting! Well, let's see:
>
> The polling code (including the code that schedules the cpu
> between kernel and userland) is written upon the assumption
> that only one
> polling loop is active. Now, because polling can occur in the idle
> loop or right after hardclock, and there are multiple concurrent
> idle_loops in the SMP cases, and the locking is just not there.
>
> Probably, the reason it works for you is that likely there is only
> one instance of hardclock handler, _and_ the idle_loop code is
> different for the UP and SMP cases, and I only provided polling
> hooks for the former. So you are limited to doing polling only in
> the assigned fraction of CPU, and the CPU(s) will not use any extra
> CPU for polling even if available.
I think since idle_poll does:
int
idle_poll(void)
{
if (poll_in_idle_loop && poll_handlers > 0) {
int s = splimp();
in_polling = 1;
enable_intr();
ether_poll(poll_each_burst);
disable_intr();
splx(s);
vm_page_zero_idle();
in_polling = 0;
return 1;
} else
return vm_page_zero_idle();
}
that it won't matter since splimp() is held.
Am I missing something? I should be able to place a call
to _idle_poll in the swtch.s idle_loop() for SMP?
not sure why it enables interrupts before calling poll,
and disables them afterwards?
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